Heat transfer through covalent bonding of thermal interface material

ABSTRACT

A thermal interface material may be covalently bonded to a bottom surface of a heat dissipating device and/or a backside surface of a heat generating device. The heat dissipating device may be thermally coupled to the heat generating device, the thermal interface material disposed between the bottom surface of the heat dissipating device and the backside surface of the heat generating device. The thermal interface material may comprise a polymer material with thermally conductive filler components dispersed therein. For one embodiment, the thermally conductive filler components may be covalently bonded together. For one embodiment, the thermally conductive filler components may be covalently bonded with the polymer material.

FIELD OF THE INVENTION

[0001] The present invention relates to integrated circuit (IC) packagetechnology and more particularly to improved heat dissipating fromintegrated circuit packages.

BACKGROUND OF THE INVENTION

[0002] As integrated circuits (ICs) become smaller and faster, theamount of heat generated per square inch may increase accordingly.Therefore, one of the challenges presented to IC package designers is todissipate heat. An IC package typically includes an IC mounted on apackage substrate. A heat dissipating device, such as an integrated heatspreader (IHS) or a thermal plate, may be coupled to a backside surfaceof the IC, in an effort to remove heat from the IC. Imperfections in themating surfaces of the IC and the heat dissipating device may result insmall gaps of air between the devices. Because air is a poor conductorof heat, these gaps may serve as a barrier to heat transfer. A thermalinterface material (TIM) with a higher thermal conductivity than air maybe disposed between the IC and the heat dissipating device in an effortto fill these gaps and enhance heat transfer.

[0003] The TIM is typically made of a polymer material in combinationwith filler components made of a thermally conductive material, such asmetal or ceramic. The polymer material may promote adhesion with the ICand the heat dissipating device and may bind the filler componentstogether. Because the polymer material typically has a low thermalconductivity, the thermally conductive filler components may provide themain path for heat transfer. Therefore, heat transfer may be dependenton physical contact between filler components and the surfaces of the ICand the heat dissipating device, as well as physical contact betweenadjacent filler components in the bulk TIM.

[0004] However, layers of polymer material may prevent direct physicalcontact between filler components and the surfaces of the IC and theheat dissipating device, which may increase contact thermal resistanceat these interfaces. Further, layers of polymer material may also fillgaps between adjacent filler components which may prevent directphysical contact between the surfaces of the adjacent filler componentsand may increase bulk thermal resistance of the TIM.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 illustrates an exemplary thermal interface between anintegrated circuit (IC) and a heat dissipating device according to oneembodiment of the present invention.

[0006]FIG. 2 illustrates a flow diagram according to one embodiment ofthe present invention.

[0007]FIG. 3 illustrates an exemplary hybrid polymer according to oneembodiment of the present invention.

[0008]FIG. 4 illustrates an integrated circuit (IC) package according toone embodiment of the present invention.

[0009]FIG. 5 illustrates a flow diagram according to another embodimentof the present invention.

DETAILED DESCRIPTION

[0010] In the following description, numerous specific details are setforth, such as material types and ranges, in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of skill in the art that the invention may be practiced withoutthese specific details. In other instances, well-known elements andprocessing techniques have not been shown in particular detail in orderto avoid unnecessarily obscuring the present invention.

[0011] The present invention utilizes covalent bonding to reduce thermalresistance across an interface between a heat generating device, such asan integrated circuit (IC), and a heat dissipating device. For oneembodiment, covalent bonds may be formed at an interface between athermal interface material (TIM) and the heat generating device and/orthe heat dissipating device. Covalent bonding may result in strongerbonding between atoms and molecules which may result in more effectiveheat transfer than may be possible using traditional methods that relyon physical contact for heat transfer. For one embodiment, covalentbonding between thermally conductive filler components and apolymer-based material may also reduce bulk thermal resistance of theTIM.

[0012]FIG. 1 illustrates an exemplary thermal interface 100 between anIC 102 and a heat dissipating device 104, according to one embodiment ofthe present invention. The heat dissipating device may be any suitableheat dissipating device, such as an integrated heat spreader (IHS), athermal plate, a heat pipe lid, or a heat sink. The heat dissipatingdevice may be made of any suitable thermally conductive material, suchas a metal. For one embodiment, the heat dissipating device may bealuminum or copper. A TIM 106 may be disposed between a backside surfaceof the IC and a bottom surface of the heat dissipating device to enhanceheat flow from the IC to the heat dissipating device. The TIM may be anysuitable type TIM, such as a thermal grease, thermal adhesive, a thermalgel, an elastomer, a phase change material, or a thermal gap filler.

[0013] As illustrated, the TIM may comprise a polymer material 108 withthermally conductive filler components 110 dispersed therein.Intermediate interfaces 112 and 114 may be formed between the IC and theTIM, and between the TIM and the heat dissipating device, respectively.The total thermal resistance (θ_(TOTAL)) across the interface betweenthe IC and the heat dissipating device may be defined as the sum of thecontact thermal resistance at the interface between the IC and the TIM(θ_(IC-TIM)), the bulk thermal resistance of the TIM (θ_(BULK)), and thecontact thermal resistance at the interface between the TIM and the heatdissipating device (θ_(TIM-HDD)):

θ_(TOTAL)=θ_(BULK)+θ_(IC-TIM)+θ_(TIM-HDD).

[0014] While θ_(TOTAL) for a typical interface between an IC and a heatdissipating device may usually be in the range of 0.5 to 1.5 cm²C/W, forone embodiment of the present invention, θ_(TOTAL) may be less than 0.45cm²C/W.

[0015]FIG. 2 illustrates a flow diagram 200 illustrating exemplaryoperations of a method according to one embodiment of the presentinvention. The operations of flow diagram 200 may be described withreference to the exemplary embodiment illustrated in FIG. 1. However, itshould be understood that the operations of flow diagram 200 may resultin embodiments other than the exemplary embodiment of FIG. 1.

[0016] For block 210, a TIM with increased thermal conductivity isprovided. A TIM with increased bulk thermal conductivity may havereduced bulk thermal resistance due to an inverse relationship betweenthermal resistance and thermal conductivity. For one embodiment, bulkthermal conductivity may be increased by forming covalent bonds betweenfiller components 110, such as covalent bond 120 and/or by formingcovalent bonds between filler components 110 and the polymer material108, such as covalent bond 122. Covalent bonds may be represented bydouble lines in FIG. 1.

[0017] Examples of TIMs with increased bulk thermal conductivity due tocovalent bonding include molecular composites, nanocomposites, thermallyconductive polymers, and crystalline polymers. Increased bulk thermalconductivity may be defined as a bulk thermal conductivity greater than0.5 W/mK. For one embodiment, a TIM may be provided with a bulk thermalconductivity greater than 4 W/mK.

[0018] For one embodiment, a TIM may comprise a molecular compositeincorporating a hybrid polymer with covalent bonding between thermallyconductive components. For example, FIG. 3 illustrates an exemplaryhybrid polymer 300, comprising metal or ceramic filler components 310with covalent bonds 320 between the filler components. The hybridpolymer may be produced by organometallic or organic/inorganicsynthesis.

[0019] To produce the TIM, for one embodiment, filler components may betreated to promote covalent bonding with the polymer base material. Forexample, metal or ceramic filler components may be treated with anoxidizing agent or with a coupling agent, such as silane, to form OH, NHand/or COOH groups, which may react with a polymer material, such as aCOOH terminated polymer material.

[0020] For block 220 of FIG. 2, the TIM is covalently bonded to a bottomsurface of a heat dissipating device and/or a backside surface of a heatgenerating device. For example, the TIM may be covalently bonded to thebottom surface of the heat dissipating device 102 via covalent bonds 124and/or the backside surface of the IC 104 via covalent bonds 126.Covalently bonding the TIM may reduce contact thermal resistance at theintermediate interfaces 112 and 114 between the TIM and surfaces of theIC and the heat dissipating device, respectively.

[0021] For example, while a typical contact thermal resistance at one ofthe intermediate interfaces may be in the range of 0.15 to 0.25 cm²C/W,for one embodiment of the present invention, the sum of the contactthermal resistances at both intermediate interfaces may be less than0.15 cm²C/W. Various processes may be used to covalently bond the TIM toone or both of the surfaces, such as electrodeposition of a polymer oroligomer, electropolymerization of a monomer, surface grafting, andchemical treatment of the surface.

[0022] Electrodeposition of an electroactive polymer or oligomer mayutilize the heat dissipating device or IC as an electrical conductor orsemiconductor. For example, the hybrid polymer 300 illustrated in FIG. 3may be suitable for electrodeposition onto a metal surface of the heatdissipating device or a silicon surface of the IC. As illustrated, thehybrid polymer may have electroactive end groups, such as NH_(n) ⁺ andCOO⁻. Positive or negative charges may be introduced at one end of thepolymer chain, while discharges from the other end of the polymer chainmay be deposited on a surface of the heat dissipating device or IC.Electrodeposition may also produce physical bonding superior totraditional methods of adhesion, which may also reduce contact thermalresistance.

[0023] Elecropolymerization may comprise deposition of a monomer on asurface of the IC and/or the heat dissipating device. The monomer may becyclic or unsaturated. As with electrodeposition, the heat dissipatingdevice and/or the IC may be utilized as an electrical conductor orsemiconductor. Free radical, anionic, or cationic species may begenerated to initialize chain polymerization of the monomer, which mayresult in covalent bonding between the polymer and the surface.

[0024] Surface graft treatment may also result in covalent bonding at asurface of the IC and/or heat dissipating device. For one embodiment,surface graft polymerization may be applied to a monomer deposited onthe surface to be treated. Alternatively, a pre-formed polymer oroligomer may be grafted onto the surface. Suitable grafting techniquesare well known in the art.

[0025] For one embodiment, a surface of the heat dissipating deviceand/or the IC may be chemically treated to generate a functional groupwhich may react with the TIM to form covalent bonds. For example, asilicon surface of the IC may be oxidized by an oxidizing agent such asKMnO₄, to generate COOH or OH which may react with a polymer-based TIM,such as an epoxy resin. As another example, a surface of the heatdissipating device may be treated to form aluminum oxide (Al₂O₃), whichmay react with a COOH terminated TIM.

[0026] For block 230 of FIG. 2, the heat dissipating device is thermallycoupled to the heat generating device with the TIM disposed between thebottom surface of the heat dissipating device and the backside surfaceof the heat generating device. For one embodiment, the TIM may bedeposited on the backside surface of the IC and the heat dissipatingdevice may then be coupled to the backside surface of the IC. Foranother embodiment, the TIM may be applied to the bottom surface of theheat dissipating device and the heat dissipating device may then becoupled to the backside surface of the IC.

[0027] For different embodiments, the different treatment processesdescribed above may be combined in various manners. For example, anelectroactive polymer may be electrodeposited on the bottom surface ofthe heat dissipating device, while the backside surface of the IC may bechemically treated to form covalent bonds with the TIM. For anotherembodiment, treating a surface of only one of the devices may reduce thetotal thermal resistance enough to satisfy heat dissipating requirementsof a package application. Treating only one device may reducemanufacturing costs. For one embodiment, reducing contact resistance mayreduce total thermal resistance enough to satisfy heat dissipationrequirements without using a TIM with increased bulk thermalconductivity. For another embodiment, using a TIM with increased bulkthermal conductivity may adequately reduce total thermal resistancewithout reducing contact resistance.

[0028]FIG. 4 illustrates an exemplary IC package 400, according to oneembodiment of the present invention. The IC package may be a flip chippackage, also known as control collapse chip connection (C4) package,having an IC 402 mounted on a package substrate 408, which may bemounted on a PC board 410. An integrated heat spreader (IHS) 404 may bethermally coupled to a backside surface of the IC with a first TIM 406disposed between a bottom surface of the IHS and a backside surface ofthe IC. The IHS may be mounted on standoffs 412 attached to the PCboard. The previously described methods may be applied to reduce thetotal thermal resistance across the interface between the IC and theIHS.

[0029] As illustrated, in an effort to further enhance heat transferfrom the IC package, a heat sink 414 may be thermally coupled to a topsurface of the IHS. The heat sink may include fins or other protrusionsto increase its surface area, which may increase its ability to removeheat from the IC package. A second TIM 416 may be disposed between thetop surface of the IHS and a bottom surface of the heat sink. For oneembodiment, the previously described methods may be applied to reducethe total thermal resistance across the interface between the IHS andthe heat sink.

[0030]FIG. 5 illustrates a flow diagram 500 illustrating exemplaryoperations of a method to fabricate an IC package according to oneembodiment. For block 510, a first TIM is covalently bonded to a bottomsurface of an IHS. For block 520, the IHS is thermally coupled to abackside surface of an IC, the TIM disposed between the bottom surfaceof the IHS and the backside surface of the IC. For block 530, a secondTIM is convalently bonded to a bottom surface of a heat sink and/or abackside surface of the IHS. For block 540, the heat sink is thermallycoupled to the IHS, the second TIM disposed between the bottom surfaceof the heat sink and the top surface of the IHS.

[0031] In the foregoing description, the invention has been describedwith reference to specific exemplary embodiments thereof. It will,however, be evident that various modifications and changes may be madethereto without departing from the broader spirit or scope of thepresent invention as defined in the appended claims. The specificationand drawings are, accordingly, to be regarded in an illustrative ratherthan a restrictive sense.

What is claimed is:
 1. A method comprising: covalently bonding a thermalinterface material to a bottom surface of a heat dissipating deviceand/or a backside surface of a heat generating device; and thermallycoupling the heat dissipating device to the heat generating device, thethermal interface material disposed between the bottom surface of theheat dissipating device and the backside surface of the heat generatingdevice.
 2. The method of claim 1, wherein covalently bonding the thermalinterface material to a bottom surface of a heat dissipating deviceand/or a backside surface of a heat generating device compriseselectropolymerization of a monomer.
 3. The method of claim 1, whereincovalently bonding the thermal interface material to a bottom surface ofa heat dissipating device and/or a backside surface of a heat generatingdevice comprises electrodeposition of an electroactive polymer.
 4. Themethod of claim 3, wherein electrodeposition of an electroactive polymercomprises electrodeposition of an electroactive polymer on a metalsurface of the heat dissipating device.
 5. The method of claim 3,wherein the electroactive polymer has an electroactive end group —NH_(n)⁺.
 6. The method of claim 3, wherein the electroactive polymer has anelectroactive end group —COOH or —COO—.
 7. The method of claim 1,wherein covalently bonding the thermal interface material to a bottomsurface of a heat dissipating device and/or a backside surface of a heatgenerating device comprises surface grafting a polymer on the bottomsurface of a heat dissipating device and/or a backside surface of a heatgenerating device.
 8. The method of claim 1, wherein covalently bondingthe thermal interface material to a bottom surface of a heat dissipatingdevice and/or a backside surface of a heat generating device compriseschemically treating the backside surface of the heat generating deviceto generate a functional group that can react with the thermal interfacematerial to form covalent bonds.
 9. The method of claim 8, whereinchemically treating the backside surface of the heat generating devicecomprises oxidizing a silicon surface of the heat generating device withan oxidizing agent.
 10. The method of claim 9, wherein the oxidizingagent is KMnO₄, and the thermal interface material comprises an epoxyresin.
 11. The method of claim 1, wherein the heat generating device isan integrated circuit and the heat dissipating device is an integratedheat spreader.
 12. The method of claim 11, wherein the thermal interfacematerial has a bulk thermal conductivity greater than 4 W/mK.
 13. Amethod comprising: applying a thermal interface material to a backsidesurface of a heat generating device and/or a bottom surface of a heatdissipating device, wherein the thermal interface material comprises apolymer material with thermally conductive filler components dispersedtherein, the thermally conductive filler components covalently bondedtogether and/or covalently bonded with the polymer material; andattaching the heat dissipating device to the heat generating device, thethermal interface material disposed between the backside surface of theheat generating device and the bottom surface of the heat dissipatingdevice.
 14. The method of claim 13, wherein the TIM comprises amolecular composite material with covalent bonding between metal orceramic filler components.
 15. The method of claim 13, comprisingproducing the thermal interface material by chemically treating metal orceramic filler components to form a functional group that can react withthe polymer material to form covalent bonds.
 16. An apparatus,comprising: a heat generating device; a heat dissipating devicethermally coupled to a backside surface of the heat generating device;and a first thermal interface material disposed between the backsidesurface of the heat generating device and a bottom surface of the heatdissipating device, the first thermal interface material covalentlybonded to the bottom surface of the heat dissipating device and/or thebackside surface of the heat generating device.
 17. The apparatus ofclaim 16, wherein the heat generating device is an integrated circuit.18. The apparatus of claim 17, wherein the first thermal interfacematerial comprises an epoxy resin covalently bonded to the backsidesurface of the integrated circuit.
 19. The apparatus of claim 16,wherein the first thermal interface material comprises a molecularcomposite material.
 20. The apparatus of claim 16, wherein the firstthermal interface material comprises a nanocomposite material.
 21. Theapparatus of claim 16, wherein the first thermal interface materialcomprises a thermally conductive polymer.
 22. The apparatus of claim 16,wherein the first thermal interface material has a thermal conductivitygreater than 4 W/mK.
 23. The apparatus of claim 16, comprising anelectroactive polymer bonded to the heat dissipating device byelectrodeposition.
 24. The apparatus of claim 16, wherein the heatdissipating device is an integrated heat spreader.
 25. The apparatus ofclaim 24, comprising a heat sink thermally coupled to a top surface ofthe integrated heat spreader.
 26. The apparatus of claim 25, comprisinga second thermal interface material disposed between the top surface ofthe integrated heat spreader and a bottom surface of the heat sink, thesecond thermal interface material covalently bonded to the bottomsurface of the heat sink and/or the top surface of the integrated heatspreader.